The present invention relates to a detection device of an electronic timepiece which drives a stepping motor with low power consumption constantly even if the timepiece is subjected to an external alternating magnetic field.
In order to drive a timepiece stepping motor with less power consumption, such as an ultra micro stepping motor of an electronic wristwatch, the so-called correction driving method has been proposed. The correction driving method is a method for driving a motor with a low power consumption when the stepping motor drives normally, and for driving the motor with more power consumption than usual when the motor rotor does not rotate normally for some reason or other, without delay.
When using the correction driving method, it is important to detect rotation and non-rotation of the rotor and to assure the rotation of the rotor against adverse external conditions, such as magnetic fields in comparison with the conventional fixed pulse driving method.
FIG. 1(A) shows an embodiment of a bipolar stepping motor used in the conventional electronic watch for driving the hands and which can be used as well in the present invention, and FIG. 1(B) shows an embodiment of alternate polarity driving pulses conventionally used for driving the stepping motor of this structure.
By applying the driving pulses of alternate polarity as shown in FIG. 1(B) to a coil 3, a stator 1 is magnetized and a rotor 2 is rotated stepwise at 180.degree. C. increments by repulsion and attraction of the stator 1 and the magnetic poles of the rotor 2.
The length (pulse width) of the driving pulses applied to the coil has been determined at the width to assure the rotation of the motor under any conditions of a watch. In order to assure the rotation of the motor, it is necessary to make the pulse width longer than that normally needed to drive the motor under normal load conditions so that sufficient power is available to drive the motor under greater load conditions, such as for operating a calendar mechanism, when there is an increase in the internal resistance of a battery, when there is a reduction in the battery voltage at the end of the battery life, and the like.
Accordingly, the following driving method of the stepping motor has been proposed. Namely, the stepping motor is driven by a pulse having a short pulse width which produces a small torque normally sufficient to drive the motor, and when the stepping motor temporarily stops rotating due to a heavy load, the stepping motor is driven again by a pulse having a longer pulse width which produces a sufficiently large torque to drive the motor even under the heavy load condition. However, it is difficult to provide particular detection elements, such as a mechanical contact, a Hall effect element and the like, for detecting rotation and non-rotation of the rotor since a reduction in overall watch size and a low cost are required.
Accordingly, the rotation and non-rotation of the rotor is detected taking advantage of the feature that there exists a difference in voltages induced by the oscillation of the rotor between the rotor being rotated and not rotated, after the driving pulse is applied.
FIG. 2 shows a driver detection circuit of the stepping motor according to the conventional type and the present invention. In the circuit, inputs of N channel FET gates (referred to as an N gate hereafter) 4b, 5b and inputs of P channel FET gates (referred to as a P gate hereafter) 4a, 5a are respectively separated and the N gates 4b, 5b and the P gates 4a, 5a are simultaneously OFF. The circuit comprises detection resistors 6a, 6b for detecting rotation and nonrotation of the rotor 2 and N gates 7a, 7b for switching on the detection resistors.
FIG. 3 shows a time chart of the conventional correction driving method. When a voltage is applied across the coil, a current flows in the coil through a current passage 9 in FIG. 2 during a time interval "a" in FIG. 3. Subsequently, during a time interval "b" in FIG. 3, the circuit is switched to a closed loop 10 including the detection resistor 6b in FIG. 2. At this time the voltage induced by the oscillation of the rotor 2 appears at a terminal 8b after the driving pulse is applied. If a non-rotation signal is detected during the time interval "b", the stepping motor is driven correctly by a driving pulse of sufficiently long pulse width to cause current to again flow in the current passage 9 in FIG. 2 so as to satisfy the specification of the watch during a time interval "c" in FIG. 3.
Referring next to the detection principle of rotation and non-rotation of the rotor, FIG. 4 shows current waveforms when the current is flowing in the coil 3 of the stepping motor whose coil resistance is 3 K.OMEGA. and number of turns is 10000 turns. The current waveform during the time interval "a" is due to the driving pulse of 3.9 msec pulse width and shows almost the same waveforms regardless of rotation and non-rotation of the rotor. The current waveforms during the time interval "b" are pulses induced by the vibration or oscillation of the rotor 2 after the driving pulse is applied, varying in a large scale under the conditions of the rotor, i.e., whether the rotor rotates or not and whether a load is connected to the motor or not. The waveform b1 during the time interval "b" in FIG. 4 shows the current waveform in case the rotor 2 rotates and the waveform b2 shows the current waveform in case the rotor 2 does not rotate. The drives detection circuit in FIG. 2 has been invented to extract the difference in currents between the rotor being rotated and not rotated in the form of a voltage waveform. The circuit is switched to the closed loop 10 during the time interval "b" in FIG. 4, whereby the current induced by oscillation or vibration of the rotor 2 flows in the direction resistor 6b and a larger voltage waveform appears at the terminal 8b than in the case when the detection resistor is not provided. Since the current flowing in the normal direction during the time interval "b" is in the reverse direction with respect to the detection resistor 6b, the voltage induced in the resistor 6b appears as a negative voltage at the terminal 8b.
However, the N gate 5b serves as a diode using VSS as an anode voltage since there is a P-N junction between the drain and P-well when the N gate 5b is in an OFF state. Therefore the negative voltage at the terminal 8b becomes a forward voltage by the N gate 5b which serves as a diode and a forward current flows in the N gate 5b. And since the impedance is low when the forward current flows in the N gate 5b, the rotor oscillation is dampened.
The relation between the operation of the rotor 2 and the detection signal will be illustrated in conjunction with FIG. 5. FIG. 5 shows the relation between the stator 1 and the rotor 2. The stator 1 is provided with inner peripheral notches 16a, 16b to determine the indexing or stepping torque and outer peripheral notches 15a, 15b to enable the stator to be formed in one piece. As shown in the art the stator may be separated at 15a and 15b to form a two-piece stator. Magnetic poles N and S assume positions rotated at about 90.degree. from the inner peripheral notches 16a, 16b under the rest condition of the rotor 2 as shown in FIG. 5(A).
FIG. 5(B) shows the condition when the driving pulse is applied to the rotor, and the rotor rotates in the forward direction as denoted by an arrow mark 17. Since the driving pulse width is no more than 3.9 msec, the pulse is OFF at the time the magnetic poles of the rotor reach in the proximity of the inner peripheral notches. In case a heavy load is connected to the motor, the rotor cannot complete forward rotation and rotates in the reverse direction as shown in FIG. 5(C). In this case the magnetic poles of the rotor pass in the proximity of the outer peripheral notches 15a, 15b and a large current is generated in the coil. However, since the circuit in FIG. 2 is in the state of the closed loop 10 at this moment, the negative voltage is present at the terminal 8b, and the forward current flows in the N gate 5b serving as the diode, and thereby the movement rotor 2 is dampened. Accordingly, the rotor 2 is decelerated rapidly and the voltage induced by the oscillation of the rotor 2 is small thereafter. On the other hand, in case a light load is connected to the motor and the rotor continues to rotate by inertia, the rotor 2 rotates in the forward direction as denoted by an arrow mark 19 as shown in FIG. 5(D). Since the magnetic flux generated by the rotor 2 at this time is in the direction meeting at a right angle with the axis of the outer peripheral notches 15a, 15b, the induced current is small in the beginning. And a large current is generated when the magnetic poles rotate to positions adjacent the outer peripheral notches 15a and 15b.
At this time, since the negative voltage is present at the terminal 8b of the closed loop 10, the rotor is dampened by the diode effect of the N gate 5b. Thereafter the rotor passes by the rest position shown in FIG. 5(A) and the voltage which is able to detect the rotation of the rotor 2 is present at the terminal 8b in FIG. 2 when the rotor restores to its rest position.
Numeral 20 in FIG. 6(A) is the voltage waveform of the terminal 8b when the rotor 2 rotates. The time interval "a" shows the period during which the driving pulse whose pulse width is 3.9 msec is applied.
The circuit which exists in FIG. 2 at the time interval a is the current passage 9 whose VDD=1.57 V. The time interval "b" shows the voltage waveform of the voltage induced by the oscillation of the rotor in the closed loop 10 in FIG. 2. The negative voltage is clamped at about 0.5 V by the diode effect of the N gate 5b and the peak of the positive voltage is 0.4 V. The waveform 21 shows the voltage waveform of the terminal 8b when the rotor 2 does not rotate and the peak of the positive voltage is less than 0.1 V. The rotation and non-rotation of the rotor is judged or detected by distinguishing between the above two peak voltages.
Though the difference between two peak voltages is small, the voltage can be easily amplified by the method mentioned below.
The normally open loops 10 and 11 in FIG. 2 are alternately closed during the time interval "b" in FIG. 6(A).
In the loop 11, since both ends of the coil 3 are shorted by the N gates 4b, 5b having an ON resistance around 100.OMEGA. a current generated by the oscillating motion of the rotor is large. However, when the loop 10 is switched on, the current flows through the detection resistor 6b for an instant due to the inductance component of the coil 3. Therefore the high peak voltage is present for an instant across the detection resistor 6b. The voltage waveform 20 at the terminal 8b induced by the rotor 2 is as shown by a voltage waveform in FIG. 6(B) when the normally open loops 10 and 11 in FIG. 2 are alternately closed. FIG. 6(C) shows the voltage waveforms 22 and 23 of FIG. 6B on an enlarged time axis. The peak voltage on this occasion is delayed about 30 .mu.sec from the instant that the loop 10 is closed. The delay of the peak voltage is caused by the capacitance between the drain and source of the N gate 5b. The detection signals are easily amplified several times by the above mentioned method and the rotation and non-rotation of the rotor 2 can be detected much more easily. Though the rotation and non-rotation of the rotor 2 can be detected by the above mentioned method, the detection method has a serious disadvantage. Namely, when the stepping motor is subjected to an external alternating magnetic field, a voltage is induced in the coil 3 by the external magnetic field and the detection resistor mistakenly judges that the rotor rotates even in case the rotor does not rotate. Therefore to prevent the stepping motor from stopping when placed in an alternating magnetic field, the anti magnetic characteristic must be improved so that the pulse width of 3.9 msec can normally drive the stepping motor. The alternating anti magnetic-characteristic is shown by curves in FIG. 7 and is less than 3 oersteds when the pulse width is 3.9 msec.
Therefore, a very close tolerance anti-magnetic structure is required to drive the stepping motor in accordance with the correction driving method in order to reduce the overall size, thickness and cost of the timepiece. However, the advantage of the correction driving method is not fully achieved due to the space requirements and cost for the anti-magnetic structure.
On the other hand, there is another driving method to vary the normal pulse width according to the load in order to further reduce the current consumption in the stepping motor. In this case, the rotor of the stepping motor is driven by a pulse having the minimum pulse width to rotate the rotor in case of a light load such as when the calendar mechanism is not being operated.
Under such conditions of minimum pulse width, the alternating anti-magnetic characteristic deteriorates even more as shown in FIG. 7. Accordingly it is necessary to strengthen the anti magnetic structure such as by using a sealed plate and the like. Therefore the primary object of this driving method which is to reduce the current in the stepping motor in order to thereby reduce the thickness and size of the timepiece is hardly achieved.